Some wireless communication systems use directional antennas having a narrow beam at the transmitter and/or receiver units. An example of these systems is microwave point-to-point links that use directional antennas at both ends of the links. Another example is a wireless backhaul system with point-to-multipoint connections. These systems may have an antenna with a wide beam at a hub and an antenna with a narrow beam at each receiver unit.
Multiple-input-multiple-output (MIMO) spatial multiplexing transmission may be implemented in wireless communications systems to increase channel capacity. For example, a 2×2 MIMO configuration provides two input beams and two output beams at the transmitter and the receiver units. The beams may originate from one, two, or more antennas. The beams contain streams of data transmitted in parallel from the antennas.
Spatial multiplexing MIMO works well when there is sufficient isolation between the beams. One way of achieving isolation is through signal diversity. Signal diversity may be achieved through polarization, spatial, or angular differences provided with respect to the beams. For example, signal diversity may be achieved by transmitting signals using two beams with orthogonal polarizations.
The isolation between transmitted MIMO signals may be reduced by environmental conditions. For example, a transmission path encountering multipath interference from a number of reflections may decrease isolation between the signals as received by beams adapted for different signal diversity (e.g., polarization, bore site angle, spatial separation, etc.). For example, part of the signal transmitted on one polarization may experience a change in its polarization in the communications path and be received on the antenna with a different polarization in the receiver unit. Lower isolation results in reduced MIMO performance.
In wireless systems with narrow beam antennas at remote nodes the antenna direction can impact the communications path, such as to cause a low receive signal strength or poor isolation between MIMO streams. Conventionally, an antenna is configured to maximize the received signal strength indicator (RSSI) on a single beam. This conventional antenna pointing method may be useful for systems with single-input-single-output (SISO) transmission. However, for systems with MIMO spatial multiplexing transmission, antenna pointing solely based on RSSI may not guarantee acceptable MIMO performance, because the RSSI does not provide any information regarding isolation of the beams. For example, in a deep non-line-of-sight (NLOS) environment, the direction with the strongest RSSI may not have sufficient isolation between two MIMO channels